1. Field of the Invention
The present invention relates to communication equipment.
2. Description of the Related Art
Transmission of optical signals through fiber-optic networks is widely used in modern communication systems. In particular, long-haul, high data-rate wavelength division multiplexed (WDM) optical transmission is an important component of optical networking. One known way to accomplish long-haul transmission is by using soliton optical pulses. Due to special non-linear optical characteristics, a soliton pulse is less susceptible to chromatic and polarization mode dispersion than, e.g., a rectangular pulse. As such, soliton pulses can provide relatively low bit error rates and therefore high reliability for optical transmission.
FIG. 1 shows a typical prior art system 100 for transmitting data using soliton pulses. System 100 is configured to convert an electronic data stream 102 into an optical signal 104. System 100 comprises a laser 106 that generates a continuous wave (CW) beam of light. This beam is fed into an optical fiber and delivered to a first electro-optic (E/O) modulator 108. Modulator 108, also called a pulse carver, is configured to generate an optical pulse train of soliton pulses based on control signals from a modulator driver 114 receiving an electrical input signal 112. Signal 112 may be a sine wave at a reference clock frequency. The output of modulator 108 is a soliton pulse train 118. Depending on the type of E/O modulator, the frequency of pulse train 118 may be equal the frequency of signal 112 or harmonically related to it. Pulse train 118, also called an optical carrier signal, is fed into a second E/O modulator 110 configured to modulate said pulse train based on control signals from a second modulator driver 116 receiving data stream 102. The output of modulator 110 is optical signal 104. In different types of transmitters not using soliton pulses, an optical carrier signal analogous to carrier signal 118 may be a different periodically modulated optical signal.
One problem with system 100 is that it requires synchronizing optical carrier signal 118 and electronic data stream 102. Such synchronization is difficult to maintain due to often occurring and, in general, poorly controllable phase drifts in E/O modulators. As a result of phase drift, carrier signal 118 and data stream 102 may become misaligned causing inaccuracies in signal 104.
FIGS. 2A–B illustrate the effect of misalignment of signals 102 and 118 on signal 104. As shown in FIG. 2A, when signal 102 is properly aligned with signal 118, modulator 110 transmits or blocks a carrier-signal pulse depending on the logical input to driver 116. However, as shown in FIG. 2B, when signals 102 and 118 are misaligned, the shape of a transmitted pulse is distorted and/or a pulse is not properly blocked. Distorted pulses do not have the correct soliton waveform required for propagation through a long-haul optical fiber. In addition, misalignment may result in the transmission of portions of carrier-signal pulses that ideally should not be transmitted. Both of these effects may result in increased bit error rates at a receiver.